Myeloperoxidase Deficiency Manifesting as Pseudoneutropenia with Low Mean Peroxidase Index and High Monocyte Count in 4 Adult Patients

Myeloperoxidase (MPO) deficiency, one of the most common inherited phagocyte defects, and may exist as a transient phenomenon in combination with some clinical condition. Hematological analyzer ADVIA 2120i is used to identify the different types of leukocytes based on their size and staining properties, and by mean peroxidase index (MPXI). When MPO deficiency is present, neutrophils may be incorrectly counted as monocytes with lower MPXI values. We encountered a few cases of MPO deficiency with abnormally high monocytes counts resulting in pseudoneutropenia. These abnormal reports could lead to a mistaken diagnosis of severe neutropenia, which could result in unnecessary therapy. Manual differential count exhibited the normal differential count in every case. Every case yielded a markedly low MPXI value below -20. In conclusion, we suggest that MPO deficiency must be considered in patients especially when abnormally high monocyte counts combined with low MPXI values are observed.